A wireless communication system in which a relay device receives signals transmitted from one or more communication devices and relays the signals on to other communication devices has been researched for many years (for example, refer to Japanese Laid-open Patent Publication No. 2011-223419).
In such a wireless communication system, if a plurality of communication devices simultaneously transmit out signals, congestion may occur at the relay device, and the relay device may become unable to receive the signals. Therefore, in case of congestion, each communication device performs signal retransmission control.
For example, such retransmission control is defined in IEEE 802.15.6 which sets communication standards for short-range wireless communication networks such as a body area network (BAN) in which communications are performed between a plurality of body-worn communication devices.
When first transmitting a signal, a communication device conforming to IEEE 802.15.6 makes a decision as to whether or not to transmit the signal with a preset transmission probability (for example, ½) at the timing that the communication device can transmit the signal. Then, if the communication device relaying the signal fails to relay the signal, the communication device transmitting the signal reduces the transmission probability (for example, to one half of the first transmission probability, for example, to ¼) at the next timing to transmit the signal. In this way, the communication device reduces the transmission probability each time it fails to transmit the signal, thereby reducing the chance of congestion due to a problem between the signal transmitted from the communication device and the signals transmitted from other communication devices.